1. Field of the Invention
The present invention relates to a method for detecting gas phase molecular species in a sample by harmonic detection absorption spectroscopy, and to a method for detecting the same in a semiconductor processing apparatus. The present invention also relates to a system for detecting gas phase molecular species in a sample by harmonic detection absorption spectroscopy, and to a semiconductor processing apparatus comprising the same.
2. Description of the Related Art
Semiconductor integrated circuits (ICs) are manufactured by a series of processes, many of which involve the use of gaseous materials. Included among such processes are etching, diffusion, chemical vapor deposition (CVD), ion implantation, sputtering and rapid thermal processing. In these processes, contact is made between a semiconductor substrate and molecular species in the gas phase. As a result of the extremely fine features of the IC devices, parts per billion (ppb) and lower levels of impurities in the gases contacting the semiconductor substrates are usually considered necessary in order to minimize yield loss. Among the molecular impurities, moisture is extremely difficult to eliminate, and it adversely affects many semiconductor manufacturing processes.
A known method for detecting molecular species is infrared absorption spectroscopy. This method is based on the measurement of infrared light absorption which occurs at specific frequencies characteristic of a given molecule. However, when this method is used to measure trace quantities of molecular species inside a vacuum chamber, the measurement precision is often limited by light absorption due to the presence of the same species outside the chamber, where the light source and detector are disposed.
Three methods for mitigating the above described problem have been proposed. The first involves making the light path outside the vacuum chamber as short as possible relative to the path inside, the second is purging the light path outside the vacuum chamber with a pure gas and the third is to place this light path under vacuum. As used herein, the term "pure gas" refers to a gas with a moisture content of essentially zero. The second and third procedures can also be combined by evacuating prior to measurement.
However, these procedures are of limited effectiveness. For example, if the partial pressure of the molecular species of interest inside the vacuum chamber is very low, significant interference can still occur despite these precautions. In particular, when the molecular species of interest is water, components outside of the vacuum chamber often outgas water at a level sufficient to interfere with the measurement. In addition to outgassing, leaks and poorly purged volumes imposed by the geometry of the path outside the chamber can also contribute to the interference. These undesired sources of molecular species are extremely difficult or impossible to completely eliminate, thereby preventing accurate measurements from being made.
In infrared absorption spectroscopy, the absorption of light occurs with a spread about the center absorption frequency which increases linearly with pressure. Therefore, the width of the light absorption due to the molecules in the higher pressure region outside of the vacuum chamber is greater than the width due to the molecules in the lower pressure region inside the vacuum chamber. Additionally, the maximum light absorption at the center frequency is directly proportional to the partial pressure of the species of interest and inversely proportional to the width of the transition. Consequently, as the total pressure of the gas outside the vacuum chamber is increased, the width of the transition also increases.
When a simple infrared absorption measurement is used, no advantage is obtained by increasing the pressure outside of the vacuum chamber, even though the width of the signal due to molecules in that region is increased, because the partial pressure of the species of interest increases proportionately (assuming the concentration of the species remains constant), the signal due to the molecules outside the vacuum chamber is not suppressed because of the cancellation of the two effects. In fact, there would be a slight enhancement the size of which is dependent upon the Doppler linewidth. As a result, the molecular species in the light path outside of the vacuum chamber cannot be negated. Therefore infrared absorption spectroscopy is not suitable for precisely measuring trace quantities of the molecular species inside the vacuum chamber.
According to another known spectroscopic method, harmonic detection spectroscopy, the greater width of the transition due to molecules outside the vacuum chamber can be advantageously used. A more general discussion of the following may be found in C. R. Webster et al. Infrared Laser Absorption: Theory and Applications in Laser Remote Chemical Analysis, Wiley, N.Y. (1988).
In the case where the water vapor is in an air or nitrogen matrix at a pressure of 1 atmosphere or higher, the shape of the absorption feature is described by the well-known Lorentz profile as follows: ##EQU1## where I.sub.0 (v) is the incident light intensity at frequency v, I(v) is the transmitted light intensity at v, P is the pressure, c is the volume concentration of water vapor, l is the length of the light path through the sample, S is the linestrength characteristic of the given absorption feature, .gamma. is the half-width of the absorption feature and v.sub.0 is its center frequency. This expression gives a maximum light absorption: ##EQU2## when the frequency of the incident light is v.sub.0.
In the case of infrared light absorption by water vapor under vacuum conditions, i.e., at a very low partial pressure (e.g., less than about 0.1 torr, with a total gas pressure in the chamber of no more than about 0.5 torr), absorption features are much narrower. Absorption feature width is determined primarily by the Doppler effect, and results from the random motion of molecules with respect to the incident light, and is described by the following expression (Gaussian lineshape): ##EQU3## In the above equation, .gamma..sub.ED .sqroot.ln(2) is the half-width of the absorption feature under the above conditions, and depends on the center frequency of the absorption frequency, the molecular mass and the temperature. The line-center signal is given by the formula: ##EQU4##
For water vapor under vacuum conditions and at room temperature i.e., about 25.degree. C., .gamma..sub.ED is approximately equal to 0.01 cm.sup.-1 for absorption of infrared light at frequencies near 7100 cm.sup.-1 (where relatively strong absorption features accessible by convenient near-infrared diode laser sources are located). For water vapor in a matrix of air or nitrogen at one atmosphere pressure, a typical value of .gamma. is 0.1 cm.sup.-1. The value of .gamma. depends on the pressure and temperature of the gaseous sample and the center frequency of the absorption feature. For a given absorption feature at constant temperature, .gamma. is approximately described by the following formula: EQU .gamma.=.gamma..sub.ED +Pb
where b is a constant. A more accurate equation has been provided by Olivero and Longbotham, but the above is sufficient for purposes of this discussion.
In order to carry out harmonic detection, the frequency of the incident light source is modulated, with a sinusoidal modulation of amplitude a and frequency .omega. so that the frequency of light at time t is given by the expression: EQU v.sub.mod (t)=v+acos .omega.t
For first harmonic detection, that component of the signal at the detector which has a frequency .omega. and the same phase as the laser modulation is selected. This can be achieved, for example, by using a lock-in amplifier or by using a mixer to combine the detector output with a sinusoidal signal of frequency .omega., whose phase is suitably adjusted using a phase shifter, and passing the mixer output through a suitable low-pass filter. A detailed description may be found in The Art of Electronics by Horowitz and Hill. This technique is well-known and is used to remove noise components with a frequency of less than .omega. from the signal.
In second harmonic detection, the component of the signal with frequency 2.omega. is selected, in third harmonic detection, the component of the signal with frequency 3.omega. is selected, and so forth.
For the case of water vapor in nitrogen or air at one atmosphere pressure, the second harmonic signal at v.sub.0 (line center) is given by the formula: ##EQU5## Similarly, for the case of water vapor under vacuum conditions, ##EQU6## These expressions were derived by G. V. H. Wilson, J. Appl. Phys. Vol. 34 No. 11 p. 3276 (1963), who also showed that the maximum value of Signal (v.sub.0) is obtained when a/.gamma. (or a/.gamma..sub.ED)=2.2 for both cases.
Second harmonic spectroscopy may be implemented either by setting the frequency of light v emitted by the light source equal to the center frequency of the absorption feature v.sub.0 or by repetitively scanning the frequency over a region which includes v.sub.0. The former method usually requires active feedback control of the light source if it is a laser diode. In the latter case, scans over the entire absorption feature of interest are obtained. In either case, it is most advantageous if the signal at v.sub.0 is primarily due to absorption by water molecules in the sample region of interest.
FIG. 1 is a plot of Signal.sub.j, which is obtained from Signal(v.sub.0) by setting .gamma.=0.1, Pl S=1 (for ease of calculation, as only relative values are of interest), c=10.sup.-6 and a=mod.sub.j, where mod.sub.j varies between 0 and 1. It can be seen from FIG. 1 that Signal.sub.j is a maximum when mod.sub.j =0.22 (i.e. 2.2.multidot.0.1), and that the signal becomes lower for smaller values of the modulation amplitude. It follows then, that for a sample under vacuum wherein the ambient outside the vacuum chamber is at atmospheric pressure, if the modulation amplitude is set to 2.2 times the width of the absorption due to the molecular species of interest inside the vacuum chamber, this modulation amplitude will necessarily be much less than the optimum value for the same molecular species outside the chamber.
However, in practice, when it is desired to detect a small partial pressure of a molecular species such as water in a vacuum chamber, and this species is present in the light path outside the chamber, a modulation amplitude which is less than the optimum value for detection of molecules inside the chamber has been used. Sub-optimum modulation amplitudes are purposely chosen as they further suppress the signal due to the higher pressure molecules outside the vacuum chamber.
The above advantages of harmonic spectroscopy for suppressing signals due to molecules at atmospheric pressure in the light path outside the chamber has been recognized by Mucha, ISA Transactions, Vol.25, No.3, p.25 (1986). Mucha further notes the existence of an optimum modulation amplitude which balances suppression of the atmospheric pressure signals with optimization of the signals due to molecules inside the vacuum chamber. In this regard, in previous work of the presently named inventors, Inman et al, Application of Tunable Diode Laser Absorption Spectroscopy to Trace Moisture Measurements in Gases, Anal. Chem., Vol.66, No.15, pp.2471-2479 (1994), the Mucha technique was implemented by applying modulation amplitudes from 1.3 to 1.8 times the linewidth due to species inside the chamber.
However, as a result of the use of sub-optimum modulation amplitudes, this technique suffers the disadvantage of a considerable signal loss by up to a factor of four when compared with the signal theoretically obtainable, resulting in a lack of sensitivity and ability to measure trace quantities of molecular species inside a vacuum chamber.
To meet the requirements of the semiconductor processing industry and to overcome the disadvantages of the prior art, it is an object of the present invention to provide a novel method for detecting gas phase molecular species in a sample by harmonic detection spectroscopy which will allow for accurate in situ determination of gas phase molecular species in a sample at a level at least as low as in the ppb range.
It is a further object of the present invention to provide a method for detecting gas phase molecular species in a semiconductor processing apparatus by harmonic detection spectroscopy, using the inventive method.
It is a further object of the present invention to provide a system for detecting gas phase molecular species in a sample by harmonic detection spectroscopy, for practicing the inventive method.
It is a further object of the present invention to provide a semiconductor processing apparatus which includes the inventive system for detecting gas phase molecular species in a sample by harmonic detection spectroscopy.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art on a review of the specification, drawings and claims appended hereto.